In 2004, US television suffered what has become, in industry shorthand, its "Janet Jackson moment", when the singer's nipple was exposed to 90 million viewers of the Superbowl. Although a federal fine imposed on the CBS network was later overturned by a court of appeal, enough viewers and advertisers expressed concern for it generally to be accepted that US broadcasters have become more cautious in areas that go far beyond celebrity brassieres.

Seven years on, British TV may be on the brink of a similar "Rihanna/ Christina Aguilera moment". Following controversy over highly sexualised routines featuring the pop stars during last year's final of The X Factor on ITV1 – an early-evening show that is known to be one of the few modern series watched by families together – David Cameron has given broadcasters here four months to improve their policing of the 9pm "watershed", the regulatory boundary before which explicit material has traditionally not been screened. The prime minister has endorsed a report by Reg Bailey – who is, counter-intuitively, chief executive of the Mother's Union – that accuses the watchdog Ofcom of being too weak with its age-appropriate controls.

Just as many American TV executives now admit to thinking, in a version of historians' way with Christ, before-Janet and after-Janet, so Cameron and Bailey clearly hope to impose an after-Rihanna attitude on our producers.

And this seems likely to be a popular campaign. Whereas most disputes over entertainment content divide by political allegiance – with liberals traditionally more tolerant than conservatives over strong language and sex scenes on screen – concern about the premature sexualisation of young people forms a broad coalition: parents, grandparents, liberals, conservatives, traditionalists and feminists.

In my own case, though generally liberal about the broadcasting of strong content with proper watersheds and advance warnings, I think there is no doubt that The X Factor catastrophically misunderstood both its timeslot and its particular audience when choreographing and dressing the guest acts for the 2010 final.

The problem is that talent shows are neither the only nor the biggest problem in this area and, by calling for a regulatory crackdown, the prime minister and the government are completely failing to understand the way in which television is now produced and consumed, especially when it comes to shows watched by the young.

Cameron almost certainly will be successful in achieving a clean-up of live talent shows and high-profile terrestrial TV series that go out close to 9pm. These, though, are a very small part of the broadcast output. There are now hundreds of digital channels, many of whose post-watershed schedules go far beyond any level of explicitness ever contemplated by the older networks and over which Ofcom has only loose control.

And, crucially, even if the watershed can be strictly imposed, it is meaningless because viewers increasingly watch shows at a time of their own choosing, through hard drives, online replay sites and box sets.

Recently, I happened to mention to a senior politician and a long-serving TV executive scenes of drunken sex in the programme Geordie Shore, a variant of Big Brother in which eight young people from the north-east share a house and are encouraged to jump on each other, an outcome accelerated by the provision of a fridge filled with booze, a hot-tub and a so-called "shag-pad".

"Good God, what channel is that on and at what time?" was the response from both of the people I told. But that reaction is, as they say, so last century. Although notionally screened at 10pm on MTV – a suitably late slot in both Ofcom and Cameron/Bailey terms – it is one of the shows most often down-loaded on sites such as iTunes.

Although these portals require buyers to tick a box acknowledging that they are over the age of 18, this defence depends entirely on honest self-declaration. Most parents of teenagers will tell you that Geordie Shore and many other post-watershed shows are being watched on computers and mobile phones at all times of day by viewers well under the age of 18.

The Cameron/Bailey emphasis on broadcasting border patrol fails to acknowledge that time and place are becoming ever more irrelevant to television viewing. The watershed rules assume a television set in a living room that is watched, from, say, 4pm until midnight, in different combinations of family members, their availability decided by the time they come in from school or work or go to bed.

Now, though, younger audiences especially are likely to see shows online or on DVDs from box sets that they have either bought or, more likely, borrowed or copied. In this context, persuading the BBC, ITV or Channel 4 to go easy around 9pm is like trying to stop terrorism by increasing sniffer-dog patrols at airports: the public may be reassured, but the terrorists will simply achieve their ends another way.

And this is where we come to the fluttering parental heart of the matter. Arguments about the television watershed always turn on the question of how parents should and do control their children.

The significance of 9pm as the TV watershed is that this is the time that, for many decades, has been considered best for well-behaved children in a well-run home to be in bed, ready for school the next day. A majority of those reading this will have memories, from either side of the age divide, of negotiations over staying up a bit longer.

But the watershed required parental discipline and presence. So, ever since the 1960s, when Mrs Mary Whitehouse and the National Viewers and Listeners Association became the most feared and effective lobbyists in the history of British broadcasting, moralists have raised the spectacle of child viewers in an empty home vacated by working or feckless parents. Or, as became a fashion from the 1980s onwards, of younger family members having their own sets in their bedrooms.

As a result, Mrs Whitehouse and her successors, supported by sympathisers in the press, attempted to impose a sort of mezzanine-watershed, in which the most contentious scenes were screened long after 9pm, rather than a free-for-all beginning the moment that the clock-hands formed the necessary right-angle. And, indeed, Ofcom regulations do suggest a gradual introduction of tougher stuff and pre-broadcast warnings will often flag up that a show contains sex or swearing "from the outset".

It is futile now, though, to imagine, as Cameron and Bailey apparently do, a kindly but stern mum or dad murmuring "time to turn in now, Johnny" as the slaying, swearing or shagging counts begins to rise. We can do it – I have done it – but each small victory for intervention is likely to be surrounded by waves of daily defeats we never know about, through the multiplicity of other media.

As injuncting celebrities have discovered, democratising digital technology is essentially uncensorable. In the case of TV shows, because so many children have their own computers and phones and numerous friends who also will, the only effective way to control youthful viewing would be to conduct the sort of surveillance operation undertaken by Robert De Niro's retired CIA agent in Meet the Fockers, a title that itself celebrates the general liberation of mainstream entertainment.

It would be necessary not only to maintain a fearsome regime of PIN-protection and random checks on hard drives but also to ensure that our offspring only associated with people whose guardians were equally beady. Forget watersheds: this is a vision of a parent-child relationship that is more like waterboarding.

And, even in the areas of conventional scheduled broadcasting that can be subjected to time-sensitivity, there is the difficulty of judging what public taste is. Coincidentally, there has been a separate campaign in recent days for radio to become subject to a watershed. For reasons that presumably came from the perception that visual images are more threatening than audio – a view perhaps rooted in the history of pornography – the wireless was never given a moral clock-face in the way that the box was.

But now, seeking to make the case for a child-adult barrier in radio, the Mail on Sunday and Daily Mail have severely criticised the BBC for broadcasting on The News Quiz, a lunchtime show, a joke in which Sandi Toksvig accused the Conservative party of having "put the 'n' into the word 'cuts'".

In the right-leaning press, this gag, and a BBC executive's defence of it, have been cited, in language very reminiscent of Shapiro's book, as evidence of a radicalising liberal elite in control of broadcasting, who will not rest until the language now expected of rappers becomes acceptable from vicars. These potty-mouthed leftists are accused of trying to smuggle taboo words into a genteel wireless series.

Yet, proving the complexity of these disputes, the word complained of was spoken in far less disguise on another Radio 4 show several years ago by a performer routinely described as a national treasure. During the "new words" round on I'm Sorry I Haven't A Clue, Stephen Fry neologised "countryside" as "the murder of Piers Morgan". The audience laughter greeting this explicit pun on a generally unspeakable expletive includes what sounds like the full-throated roar of Middle England's approval.

And, even apart from such anomalies of reaction, a radio watershed would be as pointless as the television one in attempting to prevent the spread of the gag, which was easily available online any time a listener wanted to hear it.

Both cases demonstrate that the problem with all attempts at moral enforcement in broadcasting now is that they are based on an out-dated model of viewing and listening: of a family sitting round a TV set or wireless, switching on a programme at the time the programme-makers decided. Obsessed with times on the clock, the clean-up campaigners need to pay more attention to the date.